Bmp

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Knowledge Byte In this section, you will learn about:

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Declaring relationships in BMP entity beans Object-to-relational mapping in EJB

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Declaring Relationships in BMP Entity Beans • •

The bean developer writes the code for defining and managing relationships between BMP entity beans. Various types of the relationships that can exist between BMP entity beans are: • One-to-One Relationships • One-to-Many Relationships • Many-to-Many Relationships

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Declaring Relationships in BMP Entity Beans (Contd.) •

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One-to-One Relationships: • Specify that each instance of an entity bean is related to a single instance of another entity bean. • For example, in a banking application, each account may only have one particular loan associated with it. This means that the relationship between the Account and Loan entity beans is one-to-one. One-to-Many Relationships: • Specify that each instance of an entity bean is related to several instances of another entity bean. • For example, in a banking application, the relationship between the Customer and Account entity beans is one-to-many. This means that a customer can have more than one account in a bank but one account can belong to only one customer.

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Declaring Relationships in BMP Entity Beans (Contd.) •

Many-to-Many Relationships: • Specify that multiple instances of an entity bean are related to several instances of another entity bean. • For example, in a banking application, an account can be shared between multiple customers. In addition, one customer can have several accounts. As a result, the relationship between the Account and the Customer entity beans is many-to-many. • Can either be: • Fake many-to-many relationship • True many-to-many relationship

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Declaring Relationships in BMP Entity Beans (Contd.) •

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Fake many-to-many relationship: • Is implemented by using an association table. • For example, you can create the CustomerAccount table as an association table linking the Customer and Account tables. True many-to-many relationship: • Is implemented using the ejbLoad() method, which invokes a finder method by using the home interface of the other entity bean in the relationship. The finder method uses the primary key of the other entity bean as an argument.

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Declaring Relationships in BMP Entity Beans (Contd.) •

Directionality: • It refers to the direction in which you can navigate a relationship. • Relationships can be: • Unidirectional • Bi-directional

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Declaring Relationships in BMP Entity Beans (Contd.) •

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Unidirectional relationship: • Specifies that only one BMP entity bean consists of the get and set methods and a field that references the other BMP entity bean. For example, in a unidirectional relationship between two entity beans A and B, a client can only navigate from A to B and not from B to A. Bi-directional relationship: • Specifies that each BMP entity bean consists of one field pointing to the other BMP entity bean in the relationship. For example, in a bi-directional relationship between two entity beans A and B, a client can navigate from A to B and from B to A. • Specifies that each entity bean that participates in the bidirectional relationships consists of the get and set methods.

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Object-to-Relational Mapping in EJB •

Object-to-relational mapping: • Is the method of mapping an object to a relational database. • Enables you to store Java objects in a relational database, such as SQL Server or Oracle. • Enables you to issue queries for retrieving particular information. • Can be performed using the following two methods: • By providing the code in your application using a database access API, such as JDBC. • By using an object-relational mapper, such as Sun's JavaBlend.

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From the Expert’s Desk In this section, you will learn:

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Best Practice on: • Choosing Between HTTP Session Object and Stateful Session Bean to Maintain Client State Tips and Tricks on: • Optimizing the Performance of Stateful Session Beans • Optimizing the Use of JDBC in BMP Entity Beans FAQs on session beans

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Best Practices Choosing Between HTTP Session Object and Stateful Session Bean to Maintain Client State • •

While interacting with an enterprise application, the operation performed by the client during a session may require storing some client-specific information. The following methods are used to maintain the client state in an application: • HTTP Session Object • Stateful Session Beans

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Best Practices Choosing Between HTTP Session Object and Stateful Session Bean to Maintain Client State (Contd.) •

HTTP Session Object: • Is a simple object that can be used for storing client-specific information for a particular session. • Has the following advantages: • Requires less resources for maintaining the state in a client. • Requires less time in maintaining the client state. • Enables the clients using Servlet and JSP to maintain the client state easily.

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Best Practices Choosing Between HTTP Session Object and Stateful Session Bean to Maintain Client State (Contd.) •

The HTTP Session Object has the following disadvantages: • Client state is lost if a client restarts or logs-out from the Web browser. • Client is allowed to change its system while accessing an enterprise bean.

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Best Practices Choosing Between HTTP Session Object and Stateful Session Bean to Maintain Client State (Contd.) •

Stateful Session Beans: • Can be used to maintain the client state. • Store the conversational state of an individual client with the enterprise bean. • For example, in an online bookshop application, a stateful session bean is used for storing the names of books selected by a customer. The names stored are retained even if the client calls other methods of the bean.

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Best Practices Choosing Between HTTP Session Object and Stateful Session Bean to Maintain Client State (Contd.) •

Stateful Session Beans have the following advantages: • Enable the Web-based and non-Web-based clients to store clientspecific information. • Improve scalability of an application by using a shared pool to store the bean instances. • Provide support for transactions, security services, and RMI. • Allow a client to perform multiple operations in a single HTTP client request.

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Tips and Tricks Optimizing the Performance of Stateful Session Beans •

The measures to optimize the performance of a stateful session bean are: • Define an optimal cache size • Define an optimal time out value for each bean instance • Remove a stateful session bean, explicitly • Declare transient variables

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Tips and Tricks Optimizing the Use of JDBC in BMP Entity Beans • •

You use the JDBC packages, java.sql and javax.sql, to connect to a database. JDBC provides a connection pool that: • Consists of several open database connections. You can specify the minimum and maximum number of open connections in the connection pool. • Helps in optimizing performance, as you do not require establishing a new connection with the database.

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Tips and Tricks Optimizing the Use of JDBC in BMP Entity Beans (Contd.) •

When you use JDBC in the BMP entity beans, the following criteria should be considered: • Use batch transaction in order to execute multiple statements on a connection. • Use the PreparedStatement object to execute a statement multiple times. • Retrieve data from a database iteratively, in small amounts, instead of retrieving the entire data at one go. • Store the read-only and read-mostly data from a table in a cache. • Use the batch update feature of the statement object when you need to issue several queries to a database simultaneously. • Specify the appropriate isolation level according to the requirements of the application.

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Tips and Tricks Optimizing the Use of JDBC in BMP Entity Beans (Contd.) • •

You need to choose the right type of driver to optimize the use of JDBC driver in the BMP entity beans. When selecting a driver, the following criteria should be considered: • Use the Native protocol - All - Java driver while communicating with a database using an applet. • Use the Native API - Partly - Java driver while using a Java client to communicate with a database. • Use the JDBC-ODBC driver if the database with which you establish a connection does not support any driver. • Use the JDBC - Net - All Java driver for establishing communication between a client and a proxy server when the application is threetiered.

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FAQs •

How does a stateful session bean maintain the client conversational state? A stateful session bean stores the values of variables in a secondary storage while passivating the bean instance associated with a client.

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What is the purpose of the Passive stage in a stateful session bean life cycle? In the Passive stage, EJB Container enables an idle stateful session bean instance to service another client’s request.

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FAQs (Contd.) •

Is it necessary to override the ejbStore() and ejbLoad() methods in a BMP entity bean application if there are no delete and update options? The ejbStore() and ejbLoad() methods are used to synchronize the bean state with a database. For this reason, it is necessary to implement and override these methods.

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What is the use of ejbPostCreate() method in a BMP entity bean? The ejbPostCreate() method can be used for passing the reference of an EJB object to other entity beans.

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Challenge 1. 2. 3. 4. 5.

Stateful session beans save client’s conversational state in____________. Persistent objects in the J2EE enterprise bean applications are represented using _________beans. Entity beans are of two types, _________ and __________. The _________ method should have the same parameters as the ejbCreate() method in a BMP entity bean. The context object stores the information about the ___________ of an EJB.

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Solutions to Challenge 2. 3. 4. 5. 6.

secondary storage entity bean-managed persistence entity beans and container-managed persistence entity beans ejbPostCreate() environment